Use an external MIC on iMAC High Sierra

I bought a new iMAC and I have only 1 audio headphone output.

I bought a special cable for iMAC allowing to split the Jack 3,5 inch in a connector for the headphone and for Microphone.

I never can see in the AUDIO/SOUND setups the possibility to select an external MIC as input. I only can use the internal build in microphone. Is this normal .

I find information on the internet that it is mandatory to use a USB Microphone type. Is that correct. Why does apple then sell those special hardware Jack devices ! Not very honest if this is true.

Can someone confirm that we need indeed to use an USB external microphone if we don't like to use the build in microphone. I assume it is the same if I want to use external analogue audio. I need a USB Audio interface .

Please confirm of give me tips.